During inflammation and immune responses, leukocytes leave the blood and accumulate at the site of insult. A family of cytokines called chemokines recruit subsets of leukocytes, and are also involved in acute and chronic inflammatory processes as well as hematopoiesis. Chemokines are a subclass of cytokines, which have distinct structural features and biological effects. Their primary activity is on the chemotaxis of leukocytes, but they are also reported to have angiogenic and angiostatic effects. All chemokines bind to members of a G-protein coupled serpentine receptor superfamily that span the leukocyte cell surface membrane seven times (7-TM). The alpha or CXC chemokines are characterized by a single amino acid separating the first 2 cysteines. The beta or CC family of chemokines contain 2 adjacent cysteines. The human genes for the CC chemokines are clustered on chromosome 17q11-q12.
Chemokines are critical in the migration of leukocytes from the circulatory system to tissues, for example during inflammation processes. Most chemokines possess two major binding surfaces: a high affinity site responsible for specific ligand/receptor interactions and a lower affinity site, also called the heparin-binding or glycosaminoglycan-binding domain, believed to be responsible for the establishment and presentation of chemokine gradients on the surface of endothelial cells and within the extracellular matrix. Leukocytes are able to bind to the chemokine gradient through the high affinity receptor, which then induces remodeling of the leukocyte cytoskeleton, allowing flattening and cellular polarization. Once outside the circulation, chemokines also guide leukocytes to target tissues.
The chemokine receptor CCR4 was first identified by Power et al. (1995) J. Biol. Chem. 270:19495-19500 (Genbank accession number X85740). It was originally reported that the CC chemokines MIP-1, MCP-1 and RANTES were able to functionally interact with CCR4. However, recent data has suggested that this receptor is specific for the chemokines TARC and MDC. CCR4 mRNA is present in basophils, T cells, and monocytes, which is consistent with the finding that chemokines have been previously shown to exert a diverse range of activities on these cell types, including histamine release, chemotaxis, and Ca++ mobilization in basophils, and chemotaxis in T cells and monocytes. The expression of CCR4 on Th2 cells has been reported to be transiently increased following TCR and CD28 engagement (D'Ambrosio et al. (1998) J Immunol 161:5111-5). Activated Th1 cells also up-regulate CCR4 expression and functional responsiveness to thymus- and activation-regulated chemokine. Analysis of polarized subsets of CD8+ T cells reveals a similar pattern of chemokine receptor expression and modulation of responsiveness.
The chemokine TARC (thymus and activation-regulated chemokine) was first cloned by Imai et al. (1996) J. Biol. Chem. 271:21514-21521. TARC is expressed transiently in phytohemagglutinin-stimulated peripheral blood mononuclear cells and constitutively in thymus. Radiolabeled recombinant TARC bound specifically to T-cell lines and peripheral T cells but not to monocytes or granulocytes, and is able to elicit a chemotactic response. Expression of TARC may be upregulated by cytokines known to be produced by TH2 type T cells.
Macrophage-derived chemokine (MDC) is a recently identified member of the CC chemokine family. MDC is not closely related to other chemokines, sharing most similarity with TARC. Northern blot analysis indicates high expression of MDC in macrophages and in monocyte-derived dendritic cells, but not in monocytes, natural killer cells, or several cell lines of epithelial, endothelial, or fibroblast origin. There are also high expression levels in thymus and lower expression in lung and spleen.
Both MDC and TARC function as chemoattractants for CCR4 transfectants. Since MDC and TARC are both expressed in the thymus, it has been suggested that a role for these chemokines may be to attract CCR4-bearing thymocytes in the process of T cell education and differentiation (Imai et al. (1998) J Biol Chem 273(3):1764-1768).
Although chemokines are clearly beneficial in wound healing, hematopoiesis, and the clearance of infectious organisms, the continued expression of chemokines is associated with chronic inflammation. Therefore, this class of cytokines and/or their receptors are an attractive target for the creation of antagonists that abrogate one or more chemokine functions. It is envisioned that such antagonists could serve as a new class of anti-inflammatory drugs.